1. Field of the Invention
The present invention relates to a method of forming a thorough hole in a composite green sheet provided with a conductive layer formed therein and/or a processing method for further processing the through hole that has been formed.
2. Description of Related Art
Processes of producing a multilayer electronic component include the following process: firstly, a green sheet including an inorganic powder and an organic binder is formed, and then a conductive layer is formed on the green sheet by screen printing or the like to form a composite green sheet, which is followed by forming a through hole in the foregoing composite green sheet, in which at least a part of the through hole penetrates the conductive layer so that the conductive layer is exposed at the inner wall of the through hole. Subsequently, the through hole is filled or applied with a through hole conductor.
A plurality of the composite green sheets processed in the foregoing way are laminated together to form a multilayer body.
The through holes formed in the multilayer body are filled or applied with through hole conductors so as to constitute conduction paths and exterior electrodes. Processes used for forming a through hole in the composite green sheet include, namely, laser irradiation, punching with use of metal dies, microdrilling and the like. However, when the through hole is formed by laser irradiation, the organic binder and inorganic powder in the composite green sheet, and substances thereof that are denatured by the laser irradiation remain as waste on the surface of the inner wall and edges of the through hole. Also, in the cases of microdrilling and punching with use of metal dies, chippings of the green sheet and crumbled portions of the inner wall surface remain as waste. In any case, chippings resulting from processing the composite green sheet remain as waste on the inner wall surface and edges. When the through hole is filled with a conductive paste in this condition, filling with the conductive paste is impeded to cause defective filling, the conductive layer fails to be exposed to the inside of the through hole due to the waste, resulting in the conductive layer failing to be connected to the through hole conductor, or waste portions become detached after the filling with the conductive paste, resulting in the presence of foreign substances inside the through hole. For this reason, products that are out-of-spec in size due to irregular-shaped through holes, and problems in electrical continuity are caused.
In order to solve these problems, there has been proposed a method as shown in FIGS. 8(a), 8(b) in which a green sheet 1 is disposed on a base member 20 comprising a solid resin material such as PET (polyethylene terephthalate), PEN (polyethylene naphtahalate) or the like, and is irradiated with a laser light from the surface side of the green sheet so as to form a through hole 5 so that the laser light reaches and irradiates the base member 20 comprising PET or PEN, causing components of the base member 20 to evaporate. Waste inside the through hole is removed by the evaporation pressure. (Refer to Japanese Unexamined Patent Publication No. 2003-340818, for example.)
However, the conventional method of forming a through hole in a green sheet shown in FIGS. 8(a), 8(b) has a problem in that the amount of gas generated by evaporation of the separation film comprising PET, PEN or the like during the laser light irradiation is so small that the evaporation pressure is inadequate for removal of waste inside the through hole. In addition, there are cases where the components of PET or PEN or those denatured by the laser light remain in the through hole as waste. As described so far, waste remains in the through hole also in the method of prior art, which, as in the previous case, fails to completely preclude the problems such as defective filling during the through hole filling process, mixing of foreign substances due to the waste portions becoming detached afterward, and products that are out-of-spec in size due to irregular-shaped through holes. In particular, when a conductive layer is formed in a composite green sheet so that the conductive layer is exposed at the inner wall of the through hole, if waste of the green sheet and waste of the components of PET or PEN or waste of those that are denatured by the laser light remain in the conductive layer at the inner wall region and impede exposure of the conductive layer, the through hole conductor that fills or is applied to the through hole cannot be connected to the conductor layer.
The present invention has been devised with the foregoing problems in view, and accordingly an object thereof is to provide a method of processing a composite green sheet capable of more reliably exposing a conductive layer at the inner wall surface of a through hole by reducing waste that can remain on the inner wall surface of the through hole.